Ultra high molecular weight polyethylene (UHMWPE) fibers are frequently used in the manufacture of ballistic panels. The ballistic panels are typically formed from a plurality of woven or nonwoven sheets of UHMWPE fibers and several sheets are typically stacked together to form a panel with the required projectile stopping power. Typically, the UHMWPE fibers employed in ballistics panels must be fitted very tightly together to form an effective ballistics panel.
Unfortunately, the production of UHMWPE fibers, which are typically formed by a gel-spun process, is a very expensive process. Gel-spun polyethylene fibers are prepared by spinning a solution of ultra-high molecular weight polyethylene, cooling the solution filaments to a gel state, and then removing the spinning solvent. One or more of the solution filaments, the gel filaments and the solvent-free filaments are drawn to a highly oriented state. Gel-spun polyethylene fibers are typically formed by extruding a first solution of polyethylene in a non-volatile solvent and then cooled to form a first gel. The first gels are then extracted with a volatile solvent to form a second gel. The low throughput of the gel-spun process, the high cost of separating and recovering all of the various solvents, and the extra expense of forming the fibers into sheets via weaving or other processes makes polyethylene fibers an expensive option for the production of ballistics panels.
UHMWPE fibers have also been produced by the slitting and fibrillating of sheet material, such as shown in U.S. Pat. No. 5,200,129. This patent discloses polyethylene materials of enhanced orientation characteristics obtained by compression molding ultrahigh molecular weight polyethylene powder at a temperature below its melting point, drawing and stretching the resultant compression molded polyolefin into an oriented film, and slitting and fibrillating the oriented film to produce highly oriented UHMWPE fibers. Although the method shown in U.S. Pat. No. 5,200,129 eliminates the solvent and solvent recovery expense involved with gel-spun products, the resultant fibers must still be formed into a sheet by weaving or a similar process to form layers for ballistics applications.
Additionally, it is well known that UHMWPE fibers, whether gel-spun or slit and fibrillated from sheet material, have inherent defects that negatively affect properties such as the tensile modulus. These defects are a function of stress concentrators or microscopic stress points that result from the narrow fiber thickness and the relatively large surface area of the individual fibers. Although the polyethylene fibers individually may have a substantially high modulus value, when formed into sheets for ballistics panels the stress concentrators along the edges and boundaries of the individual fibers could lead to failure or inadequate performance of a ballistics panel constructed from the fibers.
What is needed therefore is a non-fibrous, high modulus, ultra high molecular weight polyethylene product that eliminates the stress concentrator problems inherent in fibrous ultra high molecular weight polyethylene.